One of the critical aspects of fixed disk drives and other applications involving the motion of a rotating object relative to a reference plane is the requirement that the disk or object itself be flat within very close tolerances. In the disk drive environment, if the disk(s) are not suitably flat, the data may not be recorded accurately on the disk, or the heads may crash into the disk. Similar errors can result for other types of rotating objects.
In fixed disk drives, the read/write errors which can occur because a disk is not suitably flat are caused by the aerodynamics of the head/disk assembly. In an ideal disk, the heads (which are mounted on lightweight springs) follow the contour of a rotating disk at a substantially uniform flying height. However, if the disk has a sudden variation in flatness, the springs to which one of the heads is mounted may not be able to react quickly enough to allow that head to follow the contour. More specifically, if the disk exhibits a sudden dip such that the head is unable to follow, the gap between the disk and the head increases. If the gap becomes too large, the head cannot accurately write to or read from the disk, and data errors occur.
Conversely, if the disk exhibits a rise which occurs too suddenly for the head to follow, the head may crash into the disk. While such head crashes are not in all instances fatal, they can damage both the disk and the heads, and eventually are likely to contribute to failure of the drive. It has thus been desirable to provide a disk which is as close to perfectly flat as possible.
To measure the flatness of a rotating disk has historically involved measurement of runout (or distance), velocity and acceleration for the disk. Devices which provide such measurements in at least the disk drive field are referred to as RVA testers. In rigid disk and disk drive manufacturing, the two important reference directions are the axial (parallel to the long axis of the spindle motor and perpendicular to the plane of rotation of the disk) and the radial (perpendicular to the long axis of the motor and in the plane of rotation of the disk) directions.
Numerous RVA testers are known in the prior art. For example, ProQuip offers commercially a Model SU-5800 device for testing the flatness of fixed disk media. Other similar types of devices are manufactured by Lion Precision (the DMT Series) and ADE Corporation (Microsense 2000).
The theoretical calculations for runout, velocity and acceleration are well known from Newtonian mechanics. In particular, EQU s=ut+1/2at.sup.2 ( 1) EQU v=u+at (2) EQU a=a (3)
Likewise from Newtonian mechanics, it is known that velocity can be determined from a measurement of distance by differentiating distance over time. Similarly, acceleration can be determined by differentiating velocity over time. Most, if not all, prior art devices have measured distance, and double differentiated for acceleration measurements.
Unfortunately, a significant difficulty arises with respect to devices which rely on differentiation for the velocity and double differentiation for the acceleration measurements. A single numerical differentiation tends to exacerbate any random error in the measured signals, and a double differentiation greatly exacerbates such errors. The result is that devices which use such measurement techniques are inherently inaccurate. Such inaccuracies have been accepted in the past because no alternatives were readily available. However, there has been a long felt need for an RVA tester which substantially improves upon the accuracy of the prior art.